1. Field of the Invention
This invention relates to microwave susceptors used for browning or crisping foodstuffs heated in microwave ovens and to processes for producing such susceptors.
2. Description of the Prior Art
The use of microwave ovens for preparing foodstuffs is widespread these days but a persistent problem is that food items do not "brown" or "crisp" in such ovens in the way that they do when prepared in conventional ovens. For some foodstuffs this does not matter very much, but for others, such as meats, baked goods, pizzas, fish sticks, popcorn and fish fillets, the lack of browning or crisping results in an unappealing food product.
There has been a considerable amount of attention paid to this problem and one solution has been the use of so-called microwave susceptors which are positioned adjacent to the food as it is heated. Microwave susceptors are materials which absorb some of the incident microwave energy and convert it to radiant or convected heat which browns or crisps the outer surface of the food item without affecting the microwave cooking process brought about by the remainder of the microwaves which pass through or around the susceptor.
Susceptors of this kind can be produced by vacuum depositing a thin metal layer onto a heat resistant polymer (e.g. polyester) film. This metallized plastic is then adhesively bonded to a suitable ovenable substrate or backing, such as a paperboard sheet. The metal layer, if thin enough to have sufficient electrical resistivity, is heated by currents generated in the metal by the incident microwave energy. Susceptors can be used as inserts in food packages or as food wrappings that are introduced into the microwave oven with the food items and are removed before the food items are served. Examples of this kind of susceptor are disclosed in U.S. Pat. No. 4,703,148 issued on Oct. 27, 1987 to Mikulski et al; U.S. Pat. No. 4,870,233 issued on Sep. 26, 1989 to McDonald et al; European patent publication 0,344,574 to Peshek et al and European patent publication 0,371,739 to Beckett.
U.S. Pat. No. 4,641,005, issued Feb. 3, 1987, describes microwave susceptor in which a conductive layer is formed as an extremely thin metal film deposited on a substrate protective layer by a process of vacuum vapour deposition. The protective layer is typically polyester and it is designed to be the layer most near to the food.
U.S. Pat. No. 4,267,420, issued May 12, 1981, describes a wrapping material for a food item to be subjected to microwave heating. This wrapping consists of a flexible plastic film having a very thin metal film applied thereto by vacuum vapor deposition.
Recently, the high temperature conditions in which these susceptors are used have been found to cause degradation of the components (such as polymers, adhesives and paperboards) and possibly the susceptor metal layers themselves, and there is a concern that the resulting degradation products may cause health problems if they are allowed to contaminate the food items being prepared. In studies carried out by the Federal Drug Administration of the United States, traces of benzene (a carcinogenic material) and other degradation products have been identified. There is therefore a possibility that the use of the susceptors may be limited by government regulations to temperatures below 300.degree. F. (167.degree. C.). At these temperatures the browning and crisping effects are minimal for most food items.
There is therefore a need for susceptors suitable for the browning and crisping of food that are less susceptible to degradation at elevated temperatures or which are less likely to contaminate food items if degradation does take place.